Linear polyethylenes in general, and high density polyethylenes (HDPEs) in particular, have gained wide commercial use. In particular, these polymers are used extensively in blown film applications. HDPE's commercial success is due in part to its advantageous stiffness and mechanical strength properties resulting from its high degree of crystallinity, which is typically greater than 70%. However, HDPE has relatively low ductility and toughness properties, i.e., flexibility, because it forms uninterrupted stacks of folded chain crystals during crystallization, resulting in a large average crystallite size and few tie chains. HDPE's low flexibility leads to problems that are bothersome in various industrials applications, such as this material's proneness to splitting, stress cracking, and an accelerated creep rate.
Linear low density polyethylene (LLDPE) was first introduced commercially after HPDE and does not suffer from the same low flexibility limitations. LLDPE is produced by copolymerizing ethylene with a comonomer of 1-butene, 1-hexene, or 1-octene and has a comonomer content typically less than 20 wt %. The comonomers interrupt the ethylene chain, resulting in a reduced average crystallite size and the development of more tie chains relative to HDPE, in turn resulting in a ductile, tough material. However, the enhanced flexibility of LLDPE relative to HDPE comes at the expense of the mechanical strength properties of this material. For instance, because LLDPE has a tensile modulus four to five times less than that of HDPE, thicker LLDPE blown films are necessary to deliver the sufficient film stiffness in applications as HDPE. To date, there are no polyethylene materials that have both the high mechanical strength properties of HDPE and the enhanced ductility and toughness of LLDPE.
Because of these deficiencies in individual HDPE and LLDPE polymers alone, chemical modification of each of these polymers as well as blending of these and other materials have been attempted. For example, it is possible to blend LLDPE with stiff HDPE, and/or other stiff plastics (e.g., isotactic polypropylene (iPP)), and/or inorganic fillers (e.g., silica and talc) in order to raise the stiffness of the material and allow for down-gauging of LLDPE blown films. However, none of these materials are blended into LLDPE in common practice due to the inevitable losses in elongation and in impact strength of the material. Instead, high stiffness plastics such as HDPE, iPP, or PET, are commonly separately co-extruded with LLPDE so as not to weaken the resulting LLDPE film layer. However, multiple layer co-extrusion leads to an increase in manufacturing cost while the overall reduction in final laminated film thickness is limited due to the need of adhesive or compatibilizing layers.
Blends of HDPE with a class of polyolefin based multiblock copolymers comprising crystalline and amorphous blocks, known as “olefin block copolymers” (OBCs) for end applications requiring a high degree of toughness have also been attempted, for example, in US 2011/0178245. These OBC polymers are synthesized by chain shuffling catalyst technology and consist of crystallizable ethylene/alpha-olefin blocks (hard) with very low comonomer content and high melting temperature alternating with amorphous ethylene-octene blocks (soft) with high comonomer content. OBC polymers are also described in WO 2006/101966, among others. However, the resulting blends of OBCs with HDPE do not have both the high mechanical strength properties of HDPE and the flexibility of LLDPE, likely attributable to the facts that the block length, block number, the block transition, and the crystallite sizes in the hard blocks of OBC are ill defined due to statistical shuttling nature, time required to transition from hard to soft blocks, and the random insertion of 1-octene monomer.
Other references of interest include: WO 1995/027746A1; WO 2002/066540; and WO 2013/148035A1; and K. Sakurai et al. “Blends of amorphous-crystalline block copolymers with amorphous homopolymers. Morphological Studies by electron microscopy and small angle scattering,” in 37(20) POLYMER 4443 (1996).
There is still a continuing need, therefore, for a polyethylene based material that has both the high mechanical strength properties of HDPE and the ductility and toughness, i.e., flexibility, of LLDPE.